instant tank wipeout. what could have caused it?

This is a discussion on instant tank wipeout. what could have caused it? within the Saltwater Fish Diseases forums, part of the Saltwater Fish and Coral Reef Tanks category; -->
someone help me please. my 55 gallon sw tank held a yellow tang, blue tang, 4 damsels, a lawnmower blenny, and a p. clownfish. ...

someone help me please. my 55 gallon sw tank held a yellow tang, blue tang, 4 damsels, a lawnmower blenny, and a p. clownfish. for inverts it holds a scarlet hermit crab, and a blue legged hermit crab. 40 pounds of live sand and about 10 pounds of live rock(im slowly building up) water levels are normal except for a 0.5 nitrate level.
they where all cramming themselves in a shell(all except the yellow tang who slowly hovered next to the entrance). i saw no aggressive behavior, and they all came out to eat everyday. today in the morning they wouldn't come out to eat. the yellow tang would nibble at the food but the rest wouldn't come out. so i took action and removed the conch only to find one of my beautiful domino damsels (Ying) dead. he was just pale, not a single scratch on him. all the fish went wild and wouldn't eat. just floated in a depressing way. arriving from a quick errand out of town, i found my 3 damsels and my lawnmower blenny dead. 4 fishes floating in the current. no bite marks or white spots or anything. they seem to have just died.
i have taken my water to be checked at my lfs but it came out normal. my inverts are fine and cheerful. all equipment is at prime status. and i see no sign of illness in my 3 remaining fish. any ideas on what could have happened?

The only thing I have been able to find about conch shells is that it is very hard to make sure that every little piece of flesh from the snail that lived there is gone or that nothing moved into the spiral. It seems like there might have been something in the spiral that didn't come out and rotted causing ammonia only in the shell and killed the fish that were in the shell. I know it sounds odd but I have heard of ammonia in a tank in one specific location and it never showed on a typical top water test. The ammonia was in a castle that the person had tested it in the advice from someone and low and behold it was 5 pmm inside the castle and the tank showed zero. All thanks to a handful of dead ghost shrimp.